Today's utility vehicles, such as buses and trucks, with their front control design, are characterized by spatial conditions for the accommodation of the transmission such that the transmission is necessarily a substantial distance away from the driver's seat. This distance is particularly large in the case of vehicles with under-floor or rear-mounted engines. Owing to the long and sometimes tight gearshift linkage used with mechanically shifted transmissions, exact gearshifts are often more difficult.
To allow the driver of a motor vehicle to devote his full attention to the traffic, he should so far as possible be relieved and assisted in all the activities needed for driving the motor vehicle.
Every driver knows how decisive the trouble-free operation of the gearshift system can be in difficult traffic situations. In this connection pneumatic shift-assisting devices for utility vehicles of any size can be helpful.
Servo-assistance devices known until now are partly fitted directly on the transmission itself and have a control rod and piston rod accessible from outside. The gearshift linkage is connected to the control rod. The assistance mechanism is activated by longitudinal movement of the control rod. This type of control is combined with two rod- or cable-pull shifts. A disadvantage in such cases is the sealing of the control and piston rod by bellows and the lack of lubrication. In trucks the location concerned is exposed to a lot of dirt. When the gearshift linkage ratio is changed, the beginning of the servo-assistance also changes or it has to be adapted by modifying the valve to suit the linkage ratio. The same applies to shifting aids that consist of a control valve and servo-cylinder separate from one another. The valve and cylinder are connected by spherical joints to the shift lever and a bracket, which is in turn fixed on the transmission. This arrangement has the additional disadvantage that at every shift the components move relative to the transmission and the chassis, so that the air lines connecting the valve and cylinder with one another can be perforated by chafing.
Such pneumatic shift aids with an open structure are known, consisting of a mechanical-pneumatic control element and a separate, pneumatic force element. A shift aid with an open structure is known from Loomann; Toothed-wheel transmissions; 2nd edition; Springer Verlag; 1988; p. 225. The control element is a mechanically actuated control valve, which is actuated by a shift rod. In this case the transmission of the selection movement for gearshifts takes place mechanically, directly in the transmission. During transmission of the shift movement the control valve is actuated and at the same time the manual shifting force is transmitted mechanically to the transmission by a lever. During this, the manual shifting force is additionally assisted pneumatically by a compressed air cylinder. The compressed air cylinder, a two-position cylinder with integrated hydraulic damper, forms the pneumatic force element. In this case the manual shifting force is not imaged directly proportionally. The distances between the control and force elements are long, and the structure takes up a lot of space. Damage to the compressed air lines between the control and force portions can hardly be avoided.
DE 195 39 472 A1 discloses a shifting mechanism with a servo-assistance device for a motor vehicle transmission. A control rod of the servo-assistance device is arranged axially movably within a piston and co-operates, via a gearshift linkage, with a shift lever. On the piston rod is arranged a piston which can be acted upon on both sides by a pressure medium, and the piston co-operates with means for shifting the gear-type variable-speed transmission. Axial displacement of the control rod in the piston rod can actuate control valves by means of actuating pistons. The shifting force exerted upon the control rod via the shift lever and gearshift linkage is transferred proportionally by the piston rod to the means for shifting the gear-type variable-speed transmission. During this, the two sides of the shift mechanism produce shift forces of different size, so that various gears of the vehicle's transmission can be shifted with different shifting forces. The different shifting forces are produced by different valve characteristics, given by differently sized piston faces of the actuating pistons and/or by different geometrical dimensions of the two opposed surfaces of the piston.
In the unpublished applications by the present applicant with file numbers 10 2006 006 651.0 and 10 2006 006 652.9, a shift mechanism with servo-assistance for a vehicle transmission is disclosed, which comprises means for selecting and shifting a gear of the transmission and a control rod of the servo-assistance device, which is acted upon by the manual shifting force to be assisted. In the unpublished application with file number 10 2006 006 651.0 elastic elements are provided in the shift mechanism in order to change or limit the manual shifting force that acts upon the servo-assistance device before the force passes into the servo-assistance device. In the unpublished application with file number 10 2006 006 652.9, spring elements are provided in order to change the manual shifting force that acts on the servo-assistance device within the servo-assistance device before and/or during the production of the servo-assistance force, and thereby to influence the servo-assistance device in its action.
To produce different servo-assistance forces, it is necessary to have different elastic compliances or elasticities. Depending on the design, this can be difficult to achieve and may require structural space that is often not available.
DE 10 2004 042 609 A1 describes characteristics for a servo-assistance device for a shift mechanism of a motor vehicle transmission. As a function of a manual shifting force or a shift phase the characteristic curves have sections with different gradients or proportionality to the manual shifting force. It is not disclosed in DE 10 2004 042 609 A1 how the characteristics can be realized by design.